1. Field of the Invention
The present invention relates generally to an etch electrolyte composition and method for etching anode foil to render it suitable for use in electrolytic capacitors, and to such electrolytic capacitors.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable cardioverter defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size and ability to withstand relatively high voltage. Conventionally, an electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. The electrolyte impregnated in the separator functions as the cathode in continuity with the cathode foil, while an oxide layer on the anode foil functions as the dielectric.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, increasing the surface area of the aluminum anode foil results in increased capacitance per unit volume of the electrolytic capacitor. By electrolytically etching aluminum foils, an enlargement of a surface area of the foil will occur. As a result of this enlargement of the surface area, electrolytic capacitors, which are manufactured with the etched foils, can obtain a given capacity with a smaller volume than an electrolytic capacitor which utilizes a foil with an unetched surface.
In a conventional electrolytic etching process, surface area of the foil is increased by removing portions of the aluminum foil to create etch tunnels. While electrolytic capacitors having anodes and cathodes comprised of aluminum foil are most common, anode and cathode foils of other conventional valve metals such as titanium, tantalum, magnesium, niobium, zirconium and zinc are also used. Electrolytic etching process are illustrated in U.S. Pat. Nos. 4,213,835, 4,420,367, 4,474,657, 4,518,471 4,525,249, 4,427,506, and 5,901,032.
In conventional processes for etching aluminum foil, an electrolytic bath is used that contains a persulfate oxidizing agent, such as sodium persulfate. The etching is usually followed by treatment in nitric or hydrochloric acid. Sodium persulfate is a strong oxidizing agent which can control the etch process to initiate more tunnels per unit area, and can also prevent the etch tunnel walls from being completely passivated during etch. However, sodium persulfate is thermally and electrochemically unstable and tends to decompose to sodium sulfate over time at high solution temperature. Also, sodium persulfate, if not isolated from the cathode, tends to be unduly reduced at the cathode to form sodium sulfate. Above a certain concentration, sodium sulfate is believed to be detrimental to the foil capacitance. Thus, a high standard deviation in foil capacitance can occur if the persulfate and resulting sulfate levels are not tightly controlled. Accordingly, to maintain a high capacitance yield, sodium persulfate needs to be replenished in the etch solution, and the level of sodium sulfate must be controlled (i.e., removed from the etch solution).
It would be advantageous to utilize an etch process, particularly for a direct current (DC) etch process, which provides for a high voltage, high capacitance yield using agents that are more chemically stable than persulfate.